Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on a spindle hub of a spindle motor for rotation at a high speed. Information is read from and written to each disc in a plurality of concentric tracks by a read/write head mounted on an actuator arm. The outside circumference of each disc is referred to as the “outer diameter” (OD), and the region of each disc near the spindle hub is referred to as the “inner diameter” (ID). A read/write head is said to “fly” over the disc surface as the disc rotates. When disc rotational velocity decreases, the layer of air supporting read/write head above the disc surface diminishes and the head descends toward the disc surface. However, contact between a read/write head and the disc surface can damage the magnetizable medium and the head. Furthermore, through a phenomenon called “stiction,” a read/write head can become temporarily “stuck” to the disc surface after contact with the disc surface. Stiction can damage the magnetizable medium, the read/write head, and/or the actuator arm when the disc drive system initiates disc rotation in an attempt to move the read/write head from the disc surface.
One approach to addressing this problem is to land the read/write head on a textured landing zone, preferably near the ID of the disc. Typically, data are not recorded in the landing zone, and the texturing of the landing zone surface minimizes stiction. The actuator arm is moved to an ID landing zone from data areas on the disc when the rotational velocity of the disc is decreased, thereby avoiding contact with the data areas of the disc. The read/write head is moved back to the data areas of the disc when the rotational velocity increases to allow the head to fly above the disc surface.
An alternative approach is to unload the read/write head by moving the actuator arm onto a ramp, preferably positioned outside the OD of the disc. The ramp supports the read/write head outside the diameter of the disc and prevents contact between the head and the disc surface. An actuator arm typically sweeps a 25° arc from ID to OD to access tracks on a disc; however, the ramp feature can increase the total sweep (i.e., stroke) required of the actuator arm and a voice coil motor (VCM) to approximately 50°. Furthermore, the ramp presents additional resistance to the movement of the actuator arm, because the arm must ascend the sloped surface of the ramp, which also introduces an additional friction component.
To move read/write heads from data regions to non-data locations, e.g., a ramp or landing zone, conventional actuator retract systems may utilize power provided by kinetic energy generated as the discs of the disc assembly continue spinning at disc drive power-down. Disc drive power-down may be used to identify not only power shut off to the disc drive, but also situations where power is shut off to a spindle motor supplying power to a spindle hub rotating the discs during normal disc drive operation. As power is shut off to the spindle motor, a back electromotive force (VBEMF) is produced across windings of the spindle motor as the discs spin down. The back electromotive force across the windings generates alternating currents that may be used to generate an electric field in a voice coil motor (VCM) operably connected to the actuator assembly. The electric field interacts with a magnetic field of the VCM to move the actuator assembly such that the read/write heads of the assembly are retracted away from data regions and toward non-data location on or in close proximity to the discs.
During disc drive power-down, there exists only a finite time to retract the read/write heads to either a landing zone or a load/unload ramp, depending on the disc drive configuration. Thus, it is important to move the read/write heads away from data regions on the discs at a relatively high velocity in view of the actual positional displacement of the heads. However, by retracting the read/write heads at such great velocities, the probability that particles from a head may be dislodged from the head and onto a data region on a disc is heightened. As such, a relatively high retract velocity may result in hard errors on one or more data regions on the discs.
To address this problem, conventional actuator retract systems employ an amplifier to directly measure a back electromotive force across the VCM (VVCM) as the actuator arms, and thus the read/write heads attached thereto, are retracted toward the non-data location. While these conventional systems may accomplish the task of limiting the retract velocity of the read/write heads across the discs, they are not without problems. First, the circuitry employed in conventional velocity limiting actuator retract systems is often complex in design. As such, these conventional systems are not only expensive to design and manufacture, but are associated with routine implementation errors that may be harmful to the overall operation of the disc drive. Second, transition into and out of the VVCM sampling period by this circuitry involves unnecessarily large and steep transitions in the VVCM waveform versus time, thereby resulting in irregular rates of torque applied to the actuator arms over time. These irregular rates in torque often excite components within the disc drive thereby causing the components to vibrate, which results in undesirable high levels of acoustical noise in disc drives.